In the respiratory electron transport chain, the third ATP is produced by the oxidation of:

Explanation

Cytochrome a3 is the correct option because it is the terminal component of the respiratory electron transport chain where the final transfer of electrons to molecular oxygen occurs. During this process, cytochrome a3 becomes oxidized as it donates electrons to oxygen, forming water. The energy released from this final oxidation step contributes to the proton gradient used by ATP synthase to produce the third molecule of ATP, completing the process of oxidative phosphorylation.NADH is incorrect because it donates electrons at the beginning of the chain to Complex I, not at the terminal step where the last ATP is generated.Cytochrome b is incorrect because it functions earlier in the chain within Complex III and does not provide the energy for the final ATP formation.Cytochrome c is incorrect because it only acts as a mobile carrier between Complex III and Complex IV and does not undergo oxidation linked directly to ATP synthesis.